Mesa bipolar transistor with edge contacts

ABSTRACT

A semiconductor device having a monocrystalline silicon region (3) comprising a first zone (9) and an adjacent second zone (10) and laterally enclosed by a sunken oxide layer (4) and by an overlying highly doped polycrystalline silicon layer (5). The silicon layer (5) is laterally separated by an oxide layer (6) from the silicon region (3) and adjoins the first zone (9) on a narrow edge portion of the upper surface of the region (3), this zone being of the same conductivity type as the silicon layer (5). The second zone (10) is provided with an electrode layer (11). According to the invention, the silicon layer (5) is separated from the electrode layer (11) by an oxide strip (12A) formed in a self-aligned manner and at least one doped connection zone (13) having a width determined by the oxide strip (12A) is situated between the first and the second zones and located below the oxide strip (12A). The invention further relates to a method of manufacturing this device.

This is a continuation of application Ser. No. 301,578, filed Jan. 24,1989.

The invention relates to a semiconductor device having a semiconductorbody comprising a surface-adjoining region of monocrystalline silicon,which is laterally surrounded at least in part by a sunken oxide layer,and a highly doped silicon layer disposed thereon, the silicon layerbeing laterally separated substantially entirely from the region by anoxide layer, the region comprising a doped first zone of the sameconductivity type as the silicon layer adjoining at least the edge ofthe region and an adjacent second zone, the silicon layer adjoining thefirst zone on an edge portion of the surface of the region, the secondzone being provided with an electrode.

The invention further relates to a method of manufacturing the device.

A semiconductor device of the kind described above is known from thepublication of Washio et al: "A 48 ps ECL in a Self-Aligned BipolarTechnology", ISSCC '87, pag. 58-59.

This publication discloses a bipolar transistor, which is provided in amesa-shaped semiconductor region of silicon. The mesa-shaped region islocated above a highly doped buried layer and is laterally enclosed by asunken oxide layer and a polycrystalline silicon layer disposed thereon,which adjoins a highly doped base contact zone on the upper surface ofthe mesa and is laterally separated from the mesa substantially entirelyby an oxide layer.

This known transistor has very small dimensions due to the fact thatsuccessful attempts have been made to cause the polycrystalline siliconlayer to adjoin in a self-aligned manner only a very narrow edge regionof the mesa.

However, a disadvantage of this construction is that the connectionbetween the polycrystalline base connection and the base zone isestablished solely via the base contact zone diffused from thepolycrystalline silicon. Irregularities in this edge region, which areliable to occur due to the fact that the so-called "bird's beak" edgestructure of the sunken oxide layer is not always the same, can giverise to either a poor base connection or too short a distance betweenthe polycrystalline base connection and the emitter zone. Due to thefact that the base contact zone and the emitter zone, which are bothhighly doped, adjoin each other, the emitter-base breakdown voltage canbe considerably reduced, while in given circumstances the emitter-basejunction can even extend partly in polycrystalline material, which canadversely affect the transistor properties.

It should be noted that in this application the term "polycrystallinesilicon layer" is to be understood to mean any non-monocrystallinesilicon layer, therefore also, for example, an amorphous silicon layer.

The invention has inter alia for its object to provide an improvedsemiconductor device and a method of manufacturing same, in which thedisadvantages are avoided or are reduced at least to a considerableextent.

According to the invention, a semiconductor device of the kind describedin the opening paragraph is characterized in that the silicon layer isseparated from the electrode by an oxide strip formed in a self-alignedmanner and in that at least one doped connection zone located below theoxide strip is present between the first and the second zone, theconnection zone adjoining said first and second zones and having a widthdetermined by the oxide strip.

By the use of a self-aligned connection zone, whose doping can be chosenindependently and whose width can be made very small, the aforementioneddisadvantages can be avoided without the dimensions of the transistorbeing markedly increased.

According to a first preferred embodiment, the first zone constitutesthe base contact zone of a bipolar transistor, the second zoneconstituting the emitter zone and the silicon layer constituting thebase connection of the bipolar transistor.

Another preferred embodiment is characterized in that the first zoneconstitutes the emitter zone of a bipolar transistor, the second zoneconstituting the base contact zone and the silicon layer constitutingthe emitter connection of the bipolar transistor. As a result,transistors having emitter zones of submicron dimensions can berealized, as will be explained more fully hereinafter.

The invention further relates to a particularly suitable method by whichthe semiconductor device can be manufactured with the use of a minimumnumber of masking steps.

According to the invention, this method is characterized in that

1. an insulating intermediate layer containing silicon oxide is providedon the surface of a monocrystalline silicon region and a first siliconnitride layer is provided on the intermediate layer,

2. a first silicon layer is provided on the first silicon nitride layer,

3. a silicon pattern is etched from the first silicon layer,

4. at least the edge of the silicon pattern is provided with an oxidelayer by thermal oxidation,

5. the uncovered part of the first silicon nitride layer, and thesubjacent intermediate layer are removed,

6. a depression is etched into the exposed part of the silicon region,

7. the uncovered oxide is removed,

8. the uncovered silicon is provided with a further oxide layer bythermal oxidation,

9. the remaining exposed parts of the first silicon nitride layer andthe intermediate layer are removed,

10. a second highly doped silicon layer is provided on the assembly, thesecond silicon layer being removed by planarization and etching down toa level located below that of the oxide present on the first siliconlayer,

11. the exposed silicon oxide is selectively removed by etching,

12. the exposed parts of the first silicon nitride layer are removed andat least one connection zone is formed in the subjacent parts of thesilicon region by doping,

13. the first silicon layer is selectively removed, the second siliconlayer and the connection zone are oxidized and at least one first zoneis formed by diffusion from the second silicon layer,

14. the first silicon nitride layer is removed, and

15. an electrode is provided on the surface of a second zone locatedwithin the window thus formed and bounded by the further oxide layer.

According to this method, the semiconductor device can essentially bemanufactured up to the metallization step by means of only one singlemasking step.

Preferably, the method is carried out in such a manner that after step(6) and before step (7) the uncovered silicon is provided with an oxidelayer, on which a second silicon nitride layer is formed, which is thenremoved by plasma etching from the faces parallel to the surface, andthat after step (8) and before step (9) the remaining exposed parts ofthe second silicon nitride layer are removed and the silicon surfacethus exposed is oxidized.

According to another preferred embodiment, the method is carried out insuch a manner that after step (7) and before step (8) a second siliconnitride layer is provided on the assembly, which layer is thinner thanthe first nitride layer and is removed by plasma etching from the facesparallel to the surface, and that after step (8) and before step (9) theremaining exposed parts of the second silicon nitride layer are removedand the silicon surface thus exposed is oxidized.

The invention will now be described more fully with reference to a fewembodiments and the drawing, in which:

FIG. 1 shows diagrammatically in cross-section a semiconductor deviceaccording to the invention,

FIGS. 2, 3, 4, 5, 6, 7, 8 and 9 show diagrammatically in cross-section asemiconductor device according to the invention at successive stages ofmanufacture,

FIGS. 10 and 11 show successive stages according to a first variation ofthe method in accordance with the invention,

FIG. 12 shows a stage of a second variation of the method according tothe invention, and

FIGS. 13, 14 and 15 show successive stages of manufacture of asemiconductor device according to the invention in a further embodimentof the method in accordance with the invention.

The Figures are schematic and not drawn to scale for the sake ofclarity. Corresponding parts are generally designated by the samereference numerals. Semiconductor zones of the same conductivity typeare cross-hatched in the same direction.

FIG. 1 shows diagrammatically in cross-section a semiconductor deviceaccording to the invention. The device comprises a semiconductor body 1having a monocrystalline semiconductor region 3 of silicon of a firstconductivity type adjoining a surface 2 and surrounded laterally atleast in part by a sunken silicon oxide layer 4, and a highly dopedsilicon layer 5 of a second conductivity type disposed thereon andlaterally separated substantially entirely by an oxide layer 6 from theregion 3. The semiconductor region is constituted in this embodiment bya part of an epitaxial layer grown onto a substrate 7 of the secondconductivity type. A highly doped buried layer 8 of the sameconductivity type as the region 3 is located between the region 3 andthe substrate 7.

The region 3 further comprises a doped first zone 9 of the sameconductivity type as the silicon layer 5 adjoining the edge of thisregion 3 and an adjacent doped second zone 10 of the first conductivitytype. In this embodiment, the region 3 is n-type conducting, the siliconlayer 5 and the zone 9 are p-type conducting and the zone 10 is n-typeconducting.

The silicon layer 5 adjoins the first zone 9 on an edge portion of thesurface 2 of the semiconductor region 3, while an electrode layer 11adjoining the second zone 10 is provided on the surface 2.

According to the invention, the silicon layer 5 is separated from theelectrode 11 by an oxide strip 12A formed in a self-aligned manner andat least one doped connection zone 13 located below the oxide strip 12Ais present between the first zone 9 and the second zone 10, which zone13 adjoins the first zone 9 and the second zone 10 and has a widthdetermined by the oxide strip 12A.

The connection zone 13 can be made very narrow because it is obtained ina self-aligned manner. By the use of this connection zone, whose dopingcan be freely chosen, the problems at the edge of the sunken oxidedescribed above can be avoided.

In the present embodiment, the first zone 9 constitutes the base contactzone of a bipolar transistor. The less highly doped zone 14 of the sameconductivity type constitutes the active base zone of the transistor(cf. FIG. 1). The second zone 10 of the first conductivity typeconstitutes the emitter zone (emitter connection E) and the siliconlayer 5 constitutes the base connection. The collector (C) is contactedvia the buried layer 8; this collector contact is located outside theplane of the drawing and is indicated only diagrammatically.

The semiconductor device described can be manufactured as follows.

The starting material (cf. FIG. 2) is a p-type conducting substrate 7 ofsilicon, in which a highly doped n-type layer 8 is formed by means ofion implantation and on which is then grown an epitaxial layer 3 havinga thickness of, for example, about 1 μm and a doping of, for example,about 10¹⁶ at/cm³.

According to the invention, a thin intermediate silicon oxide containinglayer 20 of silicon oxide or silicon oxynitride having a thickness of,for example, 50 nm is then formed on the surface of the monocrystallineepitaxial layer 3, while on this intermediate layer is formed a firstsilicon nitride layer 21 (thickness about 120 nm). On the latter layeris deposited an undoped polycrystalline silicon layer 22 having athickness of, for example, about 1.2 μm.

Subsequently, a pattern, for example in the form of an island, is etchedfrom this silicon layer 22 by the use of a photolithographic etchingprocess, whereupon this pattern is provided with an approximately 1 μmthick oxide layer by thermal oxidation at a temperature of 1000° C. for3.5 hours. The uncovered part of the layers 21 and 20 is then removed byselective etching successively in hot phosphoric acid and in a bufferedsolution of HF in water. Thus, the structure shown in FIG. 2 isobtained.

A depression is now etched into the exposed uncovered part of thesilicon so that a mesa-shaped region is obtained. In this embodiment(cf. FIG. 3), the mesa-shaped region only comprises parts of theepitaxial layer 3; however, the depression may also be etched throughthe layer 3.

In the embodiment of the method described here, the uncovered silicon isthen provided by thermal oxidation with an oxide layer 24, on which asecond silicon nitride layer 25 is formed having a thickness of about 50nm. This layer 25 is then selectively removed by plasma etching from thehorizontal faces parallel to the surface 2, while it is maintained onthe vertical faces (cf. FIG. 3).

The uncovered oxide (in this embodiment the oxide layers 23 and 24) isnow removed by etching, after which the uncovered silicon is provided bythermal oxidation with fresh oxide layers 4 and 26 (cf. FIG. 4). Theoxide layer 4 has, for example, a thickness of 1 μm and the oxide layer26 on the polycrystalline silicon 22 has a thickness of about 1.2 μm.

Subsequently, the remaining exposed parts of the second silicon nitridelayer 25 are etched away, a part of the silicon nitride 21 beingmaintained, after which the oxide layer 6 having a thickness of, forexample, 0.3 μm is formed by thermal oxidation (cf. FIG. 5.)

The remaining exposed parts of the first silicon nitride layer 21 and ofthe intermediate layer 20 are then removed by the etching. A secondsilicon layer 5 is then provided on the assembly, which second siliconlayer is highly p-doped during or after the provision. This siliconlayer 5 is then removed by planarization and etching by means of knowntechniques down to a level located below that of the oxide 26 present onthe first silicon layer 22. Thus, the situation shown in FIG. 6 isobtained.

The exposed silicon oxide 26 is then selectively etched away, afterwhich the exposed parts of the first silicon nitride layer 21 areremoved. Subsequently, the p-type connection zones 13 are formed in thesubjacent parts of the silicon region by implantation of boron ions. Thestructure then obtained is shown in FIG. 7.

In this embodiment, the connection zones 13 have a doping concentrationof 10¹⁸ at/cm³ and a thickness of 0.3 μm. The ion implantation iscarried out at an energy of 30 keV and a dose of 3.10¹³ boron ions percm² through the 30 nm thick oxide layer 20. It is also possible torealize the connection zones in a different manner, for example bydiffusion, in which event the oxide layer 20 is preferably removedbefore the diffusion treatment is carried out.

The first silicon layer 22 is now selectively removed by etching in aKOH solution. Due to the fact that the lightly doped silicon 22 isetched therein at a considerably higher rate than the polycrystallinehighly p-doped silicon layer 5, no etching mask is required.

Subsequently, the second silicon layer 5 and the connection zones 13 arethermally oxidized, the oxide layer 12 then being obtained, which isthicker than the layer 20. By diffusion from the highly dopedpolycrystalline silicon layer 5, the strongly p-type conducting "first"zones 9 are obtained (cf. FIG. 8). It should be noted that, if the layer20 consists of silicon oxynitride, the exposed parts of this layer mustbe etched away before the thermal oxidation is carried out.

Subsequently (cf. FIG. 9), the first silicon nitride layer 21 isselectively removed by etching. Within the window thus formed, which isbounded by the edge 12A of the oxide layer 12, the active base zone 14is formed by implantation of boron ions and then the emitter zone 10(the "second" zone) is formed by implantation of donor ions, for examplephosphorus or arsenic ions. These implantations may be carried outeither through the layer 20 or after removal of the layer 20. Otherdoping methods, for example diffusion, may also be used.

After the surface of the emitter zone 10 has been exposed, the electrode11 and connections to the layer 5 (via contact windows in the oxidelayer 12) can be provided. The collector zone can be contacted by aconnection to the buried layer 8 (via a window in the oxide layer 4).Thus, the transistor structure of FIG. 1 is obtained.

The method of manufacturing described above can be varied in many ways.

According to another embodiment of the method, there is started in thesame manner as when realizing the structure shown in FIG. 2.

Subsequently, like in the preceding embodiment, a depression is etchedin the exposed part of the silicon region. In contrast with thepreceding embodiment, however, the exposed silicon oxide 23 is thenimmediately etched away, whereupon the assembly is provided with asilicon nitride layer 25, which is removed by plasma etching from thehorizontal faces and is maintained on the vertical faces (cf. FIG. 10).The thicker first silicon nitride layer 21 is not entirely removed. Bythermal oxidation, the oxide layers 4 and 26 are then formed (cf. FIG.11).

The silicon nitride 25 is now removed entirely and the exposed thickersilicon nitride 21 is removed only in part by isotropic etching in anetching liquid, for example hot phosphoric acid. The silicon thusexposed of the region 3 and the layer 22 is then thermally oxidized, theoxide layer 6 being formed. The same situation as in FIG. 5 is nowobtained and the further procedure is effected again in the same manneras described with reference to FIGS. 5 to 9.

According to a third embodiment of the method, after the structure shownin FIG. 2 has been obtained, a depression has been etched into the layer3 and the oxide 23 has been removed, without a second silicon nitridelayer being provided, the exposed silicon is oxidized. Thus, thesituation of FIG. 12 is obtained. This structure is analogous to thatshown in FIG. 5, with the only difference that the oxide layer 6 nowpractically has the same thickness as the oxide layer 4 because noanti-oxidation layer was provided on the vertical wall of the mesa. Theprocedure of this variation of the method is further the same asdescribed with reference to FIGS. 6 to 9.

In the embodiments described thus far of the method according to theinvention, always the structure was formed which is showndiagrammatically in cross-section in FIG. 1. A bipolar transistor wasthen formed, in which event the "first" zone 9 served as base contactzone, while the "second" zone 10 formed the emitter zone of thetransistor and the polycrystalline silicon layer 5 formed the baseconnection.

The method according to the invention, however, may also be used verysuitably for the manufacture of other semiconductor devices. Forexample, with the use of the invention, inter alia a bipolar transistorhaving an emitter zone of "submicron" dimensions may be realized.

For this purpose, starting from the situation shown in FIG. 7, first thepolycrystalline silicon layer 22 is selectively etched away.Subsequently, arsenic is implanted to form in the silicon layer 5 ahighly n-doped layer (cf. FIG. 13). Then the exposed part of theintermediate layer 20 is removed by etching. Thereafter the thermaloxidation is carried out. During this thermal oxidation, the layer 5 isprovided with an oxide layer 12, but at the same time the boron and thearsenic diffuse from the layer 5 into the region 3. Due to the fact thatthe boron diffuses more rapidly than the arsenic, both a very smalln-type emitter zone 9 and an active p-type base zone 30 are thus formed(cf. FIG. 14). Due to the fact that a high dose of arsenic is used, alsothe layer 5 is fully converted to highly n-doped silicon, whichconstitutes the emitter connection.

Finally, the remaining parts of the layers 20 and 21 are etched away,after which by implantation of boron ions a highly doped p-type "second"zone 10, i.e. the base contact zone, is formed, on which again anelectrode layer 11 can then be provided (cf. FIG. 15). Thus, a bipolartransistor having a very small emitter zone 9 is obtained.

The invention is not limited to bipolar devices, but may alsoadvantageously be used in the manufacture of MOS transistors. Forexample, if in FIG. 8 the zones 9 and 13 are formed on the lefthand sideseparated from the zones 9 and 13 (by means of an additional maskingstep), these zones (9, 13) may constitute the source and drain zones ofan insulated gate field effect transistor. This gate should be providedon or at least at the area of the layers 20 and 21, if desired afterthese layers 20 and 21 have been replaced by a newly formed gate oxidelayer. In certain circumstances, the polycrystalline silicon layer 22could also be used as gate electrode. Also in this case, like in thepreceding embodiments, the "first" zone 9 is connected via the"intermediate zone" 13 to a "second" zone, which is constituted in thiscase by the channel region of a MOS transistor and is provided with theaforementioned gate electrode.

Further, in all embodiments, the conductivity types may be replaced (allsimultaneously) by the opposite types. Moreover, anti-oxidation layersother than silicon oxynitride/silicon nitride combinations may be used.

Finally, it should be noted that, in order to improve the conduction,the silicon layer 5 may be provided by means of the usual techniqueswith a surface layer consisting of a metal silicide.

We claim:
 1. A semiconductor device comprising(a) a semiconductor bodyhaving a region of monocrystalline silicon adjoining a surface of saidbody, (b) a sunken oxide layer laterally surrounding said region ofmonocrystalline silicon, (c) a highly doped silicon layer disposed onsaid sunken oxide layer and partially overlapping a top surface of saidregion of monocrystalline silicon near a lateral edge thereof, (d) anoxide layer separating substantially entirely said lateral edge of saidregion of monocrystalline silicon from said silicon layer, (e) a dopedfirst zone of said region of monocrystalline silicon, said doped firstzone being of the same conductivity type as said highly doped siliconlayer, said doped first zone being disposed at least at said top surfacenear said lateral edge of said region of monocrystalline silicon, andsaid highly doped silicon layer adjoining said doped first zone at saidtop surface of said region of monocrystalline silicon, (f) a dopedsecond zone disposed over a remaining surface of said region ofmonocrystalline silicon, said doped second zone being disposed adjacentto but separated from said doped first zone, (g) an electrode disposedon said doped second zone, (h) a self-aligned oxide strip separatingsaid highly doped silicon from said electrode, and (i) at least onedoped connection zone disposed in said top surface of said region ofmonocrystalline silicon, said at least one doped connection zone beingdisposed below said self-aligned oxide strip between said doped firstzone and said doped second zone to separate said doped second zone fromsaid doped first zone, and said at least one doped connection zonehaving a width the same as said self-aligned oxide strip.
 2. Asemiconductor device according to claim 1, wherein said doped first zoneis a base contact zone of a bipolar transistor, said doped second zoneis an emitter zone of said bipolar transistor, and said highly dopedsilicon layer is a base connection zone of said bipolar transistor.
 3. Asemiconductor device according to claim 1, wherein said doped first zoneis an emitter zone of a bipolar transistor, said doped second zone is abase contact zone of said bipolar transistor, and said highly dopedsilicon layer is an emitter connection zone of said bipolar transistor.